1. Field of the Invention
The present invention relates to an electronic component including a piezoelectric or dielectric thin film provided in an insulating film such that characteristics are improved by controlling the piezoelectric or dielectric thin film, a method for manufacturing the electronic component, and a method for planarizing the surface of the insulating film.
2. Description of the Related Art
In recent small communication devices such as cellular phones, many types of filters have been used as electronic apparatuses. The small communication devices have increasingly been required to be made small, low-profile and lightweight, and thus, the filters have also been required to be made small, low-profile and lightweight. In addition, the communication frequency has increased to the GHz range and above with increases in the quantities of information, such as image information in communication with the small communication devices.
Therefore, piezoelectric filters including piezoelectric resonators which can be made small, low-profile, and lightweight have been increasingly used as the communication frequency increases.
A known example of the piezoelectric resonators includes electrodes provided on both major surfaces of a piezoelectric substrate so as to utilize a thickness longitudinal vibration of the piezoelectric substrate. The resonance frequency of such a piezoelectric resonator is in inverse proportion to the thickness of the piezoelectric substrate, and it is thus necessary to significantly reduce the thickness of the piezoelectric substrate for use in a high-frequency region.
However, in the thin piezoelectric substrate, a practical limit of frequency in a basic mode is several hundreds MHz from the viewpoint of limits of mechanical strength and handling.
In order to solve the above-described problem, a piezoelectric thin film resonator 31 shown in FIG. 12 has been proposed. The piezoelectric thin film resonator 31 includes a thin film support member 33 having a thickness of several μm or less and provided in a Si substrate 32 by a micromachining process for partially etching the substrate 32, and a vibration unit provided on the thin film support member 33 and including a piezoelectric thin film 34, the piezoelectric thin film 34 being made of ZnO or AlN and held between a pair of lower and upper electrodes 35a and 35b defining excitation electrodes (refer to, for example, Japanese Unexamined Patent Application Publication No. 2001-168674 (Laid-Open Date: Jun. 22, 2001).
In the piezoelectric thin film resonator 31, the thin film support member 33 and the piezoelectric thin film 34 can be thinned to several μm by micromachining and a deposition method such as a sputtering process, respectively, and thus, the piezoelectric thin film resonator 31 is suitable for use in a high-frequency band of several hundreds MHz to several GHz.
However, both ZnO and Si used for the piezoelectric thin film 34 and the substrate 32, respectively, have negative temperature coefficients of Young's modulus, thereby causing the disadvantage that the resonance frequency has poor temperature characteristics.
In order to avoid this disadvantage, therefore, there is known a structure in which an insulating film made of SiO2 having a positive temperature coefficient of Young's modulus is formed on the surface of the Si substrate 32 by thermal oxidation, the Si substrate 32 is partially etched to form the thin film support member 33 including the insulating film, and the vibration unit is provided on the thin film support member 33, the vibration unit being configured such that the piezoelectric thin film 34 made of ZnO or AlN is disposed between a pair of the opposing lower and upper electrodes 35a and 35b defining the excitation electrodes.
In this structure, however, as the arithmetic average roughness (Ra) of the surface of the insulating layer increases, the arithmetic average roughness (Ra) of the lower electrode 35a provided on the insulating layer increases, and the crystallinity of the piezoelectric thin film 34 provided on the lower electrode 35a deteriorates.
Also, the resonance characteristic of the piezoelectric thin film resonator 31 deteriorates as the crystallinity of the piezoelectric thin film 34 deteriorates. Therefore, the inventors found that it is important to provide a planarized insulating film having a reduced arithmetic average roughness (Ra).